In mass spectrometry it is essential to convert the analyte molecules into ions. The most conventionally known method to create ions for mass spectrometry is electron ionization (EI), where ions are usually produced by electron impact at 70 eV. During electron ionization, positively charged molecular ions are formed having often a much higher energy then required for actual ionization which causes in most cases further fragmentation of the analyte molecules. The fragmentation also leads to numerous smaller single fragments causing an increase in the chemical background.
A softer method for sample ionization is known as chemical ionization (CI), where in an initial step a reactant gas is ionized by electron ionization and in a further step, the reactant gas ions collide with the analyte molecules thereby ionizing the analyte molecules. Typical reactant gases are, for example, methane, methanol, and i-butane. The dominating signal in the CI-mass spectrum is created by positively charged analyte-molecule ions formed by proton transfer from the reactant-gas ion to the analyte-molecule.
It is generally known that water qualifies as reactant gas. With a proton affinity of 723 kj/mol, water has, next to methane, the second strongest protonation ability of all CI-gases. Water can therefore protonate almost all organic substances. A further advantage of water is that it does not react with nitrogen, argon, oxygen and carbon dioxide because these gases have a lower proton affinity then water. Moreover, the low molecular weight of the water reactant gas ion, H3O+, allows spectra signals with a m/z ratio of 21, which makes it possible to detect low molecular substances such as, HCN, HCHO, CH3OH, or H2S, which cannot be identified with other reactant gases because of their higher molecular weight.
The problem of using water for chemical ionization is the difficulty to establish constant water vapor pressure conditions in the trap. Because the chemical ionization of an analyte is initiated by collision of the CI gas ions and the analyte molecules, pressure variations of the CI gas drastically influence the quality and quantity of the signals in the mass spectrum and precise analysis results. Because water has a low vapor pressure at room temperature, it easily condenses in the thin connection pipes as a result of minor temperature variations, finally also causing pressure variations in the trap.
Water has found some application in external chemical ionization, i.e., before entering the mass spectrometer, but this requires complex additional constructions.
Commercial ion trap mass spectrometers for chemical internal ionization do not provide the necessary conditions for the use of water as reactant gas. Because of the low vapor pressure of water, minor temperature changes may cause condensation of water drops in the pipe between the water reservoir and the entrance valve of the trap leading to an instable water pressure in the trap. In order to successfully use water as reactant gas for the internal chemical ionization of an analyte, it is essential to maintain a constant water vapor pressure in the ion trap
Because of the many advantages of water as reactant gas for the chemical ionization, there is a need of designing an ion trap mass spectrometer which can provide a stable water vapor pressure in the ion trap in order to obtain high quality and reliable spectra.